• Background "white" noise amplitude-modulated at the mains frequency.  If you were just casually listening at this amateur's QTH on the HF bands - say 40 meters - you might be forgiven in the short term for presuming that nothing was wrong.  In reality the noise floor had been elevated several "S" units by the PV system - the result sounding superficially like plain, old white noise:  Switching from SSB to AM reveals the loud "hum" that is riding on the noise - modulation that is almost "invisible" if one is listening only using SSB.

While the appearance of the above interference coincided with the activation of the neighbor's system, that fact that it disappeared at night further pointed to a PV system as the source of the QRM.

"Close-in" measurements

Placing the sense antenna right at the main inverter on the back of the neighbor's house, we wanted to take a snapshot of the spectrum at that location:

Figure 1:  0-30 MHz sampled right at the main inverter

With each horizontal division representing 3 MHz, this 0-30 MHz plot shows a high concentration of noise in the 3-9 MHz area from a location right at the inverter.

Because Figure 1 represents the spectrum at the inverter, we wondered what it would look like at one of the solar panels so we placed the sense antenna right against one of the solar panels:

Figure 2:  0-30 MHz sweep with sense antenna placed next to a solar panel

Figure 2 is in the same frequency and amplitude scale as Figure 1 - but with the "reference level" adjusted by 20 dB to move the trace "up" a bit - and we can see that the spectrum next to the panel looks quite different from that sampled right at the inverter.  This isn't unexpected as Figure 2 would likely represent more of the noise that is emitted from the DC (input) side of the optimizer whereas the spectrum represented in Figure 1 would be more likely to show that of the DC output of the optimizer plus whatever noise was riding on the conductors carrying the DC input and AC output of the main string inverter.

Although it is difficult to be sure, the 0-30 MHz plots taken from a greater distance (10 meters or more) had the general appearance of the noise spectra shown in Figure 2 more than that of Figure 1 leading me to believe that a significant portion of the QRM may be being radiated from the panels themselves rather than just the conductors going from the optimizers  to the main inverter - but certainly, both are likely involved.

Note:  For both of these plots, the RF energy from the PV system was many 10s of dB above the typical background noise floor - in this case, 40-50dB for Figure 1 and at least 30dB for Figure 2 in the area around 7 MHz.

As the 0-30 MHz sweep does not have enough resolution to visualize the narrower 200 kHz signals, the analyzer was readjusted as depicted in Figure 3 - again with the sense antenna next to the panel:

Figure 3:  From near the panel, a "zoomed in" spectral sweep showing narrowband birdies.

In this spectrum plot we can see not only the "white" noise on the floor of the sweep representing the "hummy hiss", but also the much stronger signals every 200 kHz - plus a number of weaker signals in between:  It is these signals that are the most obvious to the casual operator and appear to be unique to a SolarEdge system of this model/type.

On this same day we waited until after sunset - monitoring the groups of carriers at 7.2 MHz and hearing them "flicker" out of existence as it got dark and we re-did the "next to the panel" measurements - this time the spectrum was devoid of the 200 kHz-spaced carriers (they were no longer audible on the amateur receiver, either) and the 0-30 MHz plots were 10s of dB lower than in the daylight. 

Important:  The 2 MHz sweeps in Figures 3-7 use a resolution bandwidth of 10 kHz which is almost exactly 4 times wider than the typical SSB bandwidth of an amateur receiver of about 2.5 kHz making their apparent level above the background noise appear lower than it is actually is.

What this means is the coherent signals - such as the 200 kHz carriers - appear to be another 6 dB farther above the noise floor in an SSB bandwidth than what the analyzer plots show.

Plots from a distance

Having captured some "close-in" plots, we now had an idea as to what the signals emitted by the PV system looked like.

A few days after we made the above plots we were in a virtual meeting with the manufacturer of the PV system (SolarEdge) from the ham's shack.  Having reconfigured the feed to his main radio, we could quickly switch the feedline from the antenna feeding the radio and the spectrum analyzer.

At this time we also learned that there was a second SolarEdge system south of this amateur's QTH - about 150 feet (45 meters) away across the cul de sac - and that the neighboring system and the one across the street would but remotely shut down, in that order, to determine how much QRM was emanating from each.

While we captured 0-30 MHz plots of each stage of system shutdown, for the purposes of this article we'll show just the "narrow" plots in 2 MHz sweeps as depicted in Figure 3 as the presence of the 200 kHz signal are generally representative of the presence of the broadband noise as well and these signals were easily identifiable and now known to be indicators of QRM from this type of PV system.

First, here's the plot from the amateur's 40 meter inverted Vee antenna with both systems on:

Figure 4:  6-8 MHz plot from the 40 meter antenna showing the 200 kHz peaks - and a bit of broadband noise as well.

Figure 5:  The neighboring system off - but the one across the street still on.

As can be seen, the broadband noise floor around the 40 meter band (approximately one horizontal division below and above the marker) has dropped visibly - around 3-4 dB - and the amplitude of the carrier at 7.2 MHz has dropped about 6 dB - and the 200 kHz signals have disappeared almost entirely below about 6.5 MHz.  The system across the street was then shut off and the only remaining signals were those that happened to be on the 40 meter band.  (No trace is available for this configuration, unfortunately.)

As the 40 meter inverted Vee is oriented to favor east-west signals it was not necessarily the best candidate to test the effects of the PV system across the street, so we switched to a 20 meter antenna which was responsive in that direction and this trace shows the plot between 13 and 15 MHz:

Figure 6:  This plot of the 20 meter band and surrounding frequencies shows only propagated signals, with no sign of PV system QRM.

As both systems were off, the trace was quite clear - only showing signals that actually were on or near the 20 meter band, propagated from elsewhere in the world.  The folks at SolarEdge then turned on the system across the street with the following result:

Figure 7:  Same as Figure 6, but with only the PV system across the street activated.

The effect is clear:  In the vicinity of the 20 meter band, the appearance of rather strong signals every 200 kHz is apparent - and there is an obvious 2-4 dB increase in the noise floor indicating that this system, too, is causing harmful interference.

Readings on the radio:

It would seem that the folks at SolarEdge had worked with more than one amateur radio operator on similar issues and I was pleasantly surprised when they asked for some "S-Meter" reading comparisons with the neighbor's system on and off.  Using a calibrated signal generator, I'd already determined the signal level (in dBm) that correlated with the S-meter readings for the Icom radio - and here are the results for 40 meters:

Both systems off:

S1 (<= -84 dBm) - no carrier groups every 200 kHz.

Neighbor system on:

S4 (-78 dBm) - white noise between 200 kHz carriers.

S9 (-67 dBm) - carriers at 7.2 MHz.

This shows that at 40 meters, the degradation to noise alone was on the order of 6 dB (most Japanese radios are calibrated for 3dB per S-unit) and that the cluster of carriers on 200 kHz intervals was far more destructive, rising a bit short of 20dB out of the noise floor.

As our time with the SolarEdge folks in the virtual meeting was limited, we were not able to do similar "S-meter" tests on 20 meters, but we can use the 40 meter results along with the relative strength of the 200 kHz-spacing carriers  and correlate them with the 40 and 20 meter spectrum analyzer traces and determine that the severity of QRM from the PV system on 20 meters on the receiver would have been roughly comparable to that on 40.

Analysis of these readings and implications:

As mentioned earlier, there are two types of interfering signals produced by these SolarEdge PV systems:

• Moderate to strong clusters of carriers every "even" 200 kHz.  These are very obvious, easy to identify, and quite strong compared to the noise with a few weaker signals in-between that were also clearly audible.
  

• Background "white" noise amplitude-modulated at the mains frequency.  This is also present, but it borders on insidious as the average amateur may not be able to quantify its existence - let alone its effects - as its effects may be obscured if one only listens for it using SSB modes.

Will my radio's DSP help?

The quick answer is "No".

While you might think that modern receivers' ability to "notch out" tones might help alleviate the effects of the signals every 200 kHz, you would be wrong.

It appears that each, individual optimizer module (there is one for every solar panel) produces these signals - and being based on individual oscillators, their frequencies will be slightly different from each other meaning that instead of needing to notch just one tone, your DSP would have to notch out dozens emanating from a single PV system - and it just cannot do that!  What's worse, these carriers are also modulated by the low-rate data used to communicate to/from each, individual module which broadens their spectrum as well.

As for the "white" noise, it is unlikely that noise reduction would help much, either:  The source of this appears to be an artifact of the actual voltage converters themselves and as it is random, it is as difficult to reduce in its effects as the normal background noise of the bands.

As each optimizer module contains is own switch-mode power converter to maximize panel efficiency, they, too - like any switch-mode supply - will produce harmonic energy.  It would appear that SolarEdge uses switch-mode controllers that employ "spread spectrum" clocking so that instead of having a myriad of harmonics and birdies all throughout the RF spectrum, that energy is "smeared" all over the place making it somewhat less obtrusive.

The use of spread-spectrum clocking is very widely used these days for the reasons noted above - and for the fact that it also enables the exploitation of a quirk when a device is subjected to testing for regulatory (FCC) compliance:  Aspects of that testing specify the maximum amount of signal energy that may be present in a given bandwidth - but by "spreading" it over a much wider bandwidth, that same amount of energy would be diluted and make the readings obtained during the testing appear lower.  This is perfectly legal and commonly done - but this technique does nothing to reduce the total amount of energy radiated - only filtering can do that!

It is apparent that in this particular case, both the neighboring system and the one across the street contribute a magnitude of interference that would be considered to be "harmful" in that it is perfectly capable of submerging weak-to-moderate signals into locally-generated noise - and if such signals happened to land near a 200 kHz harmonic rather than the elevated noise floor in between the effects are >10dB more destructive.

It is also apparent that the radiated noise extends - at the very least - from the 40 meter to 20 meter bands (7-14 MHz) but the 30 MHz plots imply a significant amount of RF energy above and below this:  The limited time permitted a semi-detailed analysis of only the interference around the 40 and 20 meter bands.

After the meeting:

At the conclusion of these tests, the analyzer readings that took were forwarded to the folks at SolarEdge for their analysis - and it is still too soon to know of any conclusions that would indicate what sort of actions that they might take.  We were, however, heartened to know that they seemed to understand and were sympathetic to the plights of amateurs affected by neighboring systems that might be adversely affect amateur radio operation.

The folks at SolarEdge themselves offered the best hope of resolution:  They noted that they have a special version of PV hardware (e.g. optimizers) that has additional filtering that could be retrofitted into an existing system to reduce the potential for interference.  As this retrofit would be done on their "dime" - and it would be rather expensive - they understandably want to be sure that they have identified only systems that are of their manufacture that are causing interference.

Is a system near you?  You can listen for yourself!

Somewhat ominously, I have since tuned to 14.2 and 14.4 MHz on my mobile HF station while driving around residential and interstate roads in my local area (Salt Lake City, Utah) during my normal commute/business:  I can, in many places, hear the characteristic "roar" of narrow carriers every 200 kHz - likely from SolarEdge PV - systems as these carriers seem to disappear during the hours of darkness.

I have heard this characteristic signal even in locations that appear to be several city blocks from any structure that might be equipped with a PV system.  They may also be heard on other bands - including 40 meters - but the signals emitted on the higher bands (e.g. 20 meters) seem to be emitted with greater efficiency.

It would seem that these 200 kHz-spaced groups of carriers really get out!

"I have interference from a PV system - what should I do?"

At this point I will not reiterate in detail remediation methods that might be undertaken by a radio amateur affected by this type of PV system:  The June, 2016 QST article (link) discusses attempted mitigation using ferrite devices in detail. (Note:  This article also refers to experiences with a SolarEdge system - but the spectra of the system described there is different from what I found on the systems described here likely due to it being a now-older system.)

 I will only mention in passing that there's the possibility that a degree of mitigation may be possible with the use of "noise cancelling" antennas of the sort offered by Timewave, MFJ and others - but their utility is also somewhat limited owing to practical concerns:   Such techniques work best on distant "point sources" of interference rather than very nearby, spread-out (in terms of subtended agle) radiators in the near field.

If you have interference from a PV system, it is up to YOU to do your due diligence to determine that it is, in fact, a PV system that is causing the issues and NOT other devices in your house or those of your neighbors that is causing the problem.  If you own a PV system - or have one installed on your house - that you suspect is causing a problem, making detailed measurements with it on and off on various frequencies would be a suggested first step.

As this article relates only to the SolarEdge PV system that I investigated, I cannot possibly offer advice to another brand of system that uses other brands of equipment in regards to interference potential - but if you suspect that you or your neighbor(s) have this brand of PV system that is causing interference, I would suggest the following checks during daylight and hours of darkness as appropriate:

• Are there signals every 200 kHz?  Common frequencies where this would be observed include 3.6, 3.8, 4.0, 7.0, 7.2, 14.0, and 14.2 MHz.  This is definitely one of the hallmarks of a SolarEdge system of the same/similar model - but it similar artifacts may be produced by others.
  

• Does the "hiss" that is elevating your noise floor have an obvious "hum" to it when you switch to AM?  You can't easily hear this when you are in SSB mode.  Listen for this on frequencies in the vicinity of 60, 40, 30 and 20 meters on a frequency devoid of other signals.
  

• Does the "hummy hiss" greatly reduce when it gets very cloudy?  The "hummy hiss" - which appears to be a property of the voltage converters - seems to become more intense with increased output from the PV system.
  

• Do the 200 kHz signals and the "hummy hiss" go away after sunset and return only after sunrise?  Not unexpectedly, this is hallmark of many PV systems' noise generation.
• Be aware that some models/brands (although not the one discussed in this article) can produce RF interference if either solar illumination OR mains voltage is present and that it takes the removal of BOTH to silence them (e.g. turning of the mains breaker feeding the system at night.)

If you believe that you are being affected by a PV system, it is up to YOU to be prepared to take all appropriate measures to document the interference, do your own testing, and make repeated observations prior to reporting them to the manufacturer, a regulatory agency, club or national organization.  A few things to consider:

• Treat this as if you were causing interference to someone else.  Just as if a neighbor complained that you were causing problems to their equipment, it is incumbent on YOU to determine if the problem is on your end.  There are likely many, many devices in your house that can cause similar types of interference so be sure that you have ruled those out - and DO NOT forget that you may have devices running on UPSs or battery back-up that may still make noise even if you shut off your power.  (Many UPSs are known to be noisy in their own right!)  In other words, be certain that your house is clean before involving them as this will not only make determining the magnitude/nature of interference from a PV system easier, but it shows good will and competence on your part.
  

• Document the issue over the period of days, weeks or even months.  Many sources of interference come and go - but if it's a PV system, it will be there day in and day out.  Noting over time the consistency of the noise may give you a clue if it's some other type of device - and if it, in fact, related to a PV system: GOOD documentation will only help your case.
  

• Once you have ruled out everything else, go ahead and contact the manufacturer - but be nice!  If you are confident that your own house is in order (e.g. you have ruled out other devices) then contact the manufacturer.
• If you have been following the above steps, you will already have some documentation which makes your specific case more solid.
• The manufacturer may schedule an online meeting to discuss the issue and run tests.  Be sure that you have the ability to use Zoom or Google Groups - or find someone who does.
• If the manufacturer runs tests, they will likely turn on/off suspected systems so YOU should be ready to document changes in noise floor - and of the signals every 200 kHz (in the case of a SolarEdge system of the type investigated here).  If you have already been taking notes/documenting, you should be already familiar with your local signal environment and be able to expedite the running of these tests - and have a basis of comparison as well.
• If the manufacturer decides that they wish to help remedy your situation, remember that they may be doing it at their own expense:  It is incumbent on YOU to be cooperative, competent, courteous, accurate and honest when you are dealing with them and their requests.
• If you feel the need to do so, you may wish to enlist the help of one or more friends to help you with these tasks that may be more experienced - and having a second or even third pair of eyes on the problem is always a good idea.  If you are not comfortable doing so, I would suggest have someone else - familiar with your problem - who can talk "nerd" be your spokesperson when dealing with the manufacturer!
• You should be clear to the manufacturer to define "interference" differently from "harmful interference".  If you can just hear weak birdies that don't really cause any issues, this could be considered just plain, old "interference" and you may not get as much sympathy or action as you like.  "Harmful interference" is that which - when present - obliterates even moderately strong signals that would otherwise be quite usable and thus, they should be taken more seriously.
  

While this article is rather specific to the SolarEdge PV system as described, this is likely be applicable to other manufacturers and models in more general ways.

Good luck!

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